Conventional wireless packet communication apparatuses are adapted to proactively determine only one wireless channel to be used and detect prior to the transmission of a wireless packet whether or not the wireless channel is idle (or performs carrier sense), then transmitting one wireless packet only when the wireless channel is idle. Such transmission control by carrier sense allowed one wireless channel to be shared among a plurality of stations (hereinafter, STAs) on a time division basis ((1), International Standard ISO/IEC 8802-11 ANSI/IEEE Std. 802.11, 1999 edition, Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: (2) “Low-powered Data Communication System/Broadband Mobile Access Communication System (CSMA) Standard”, ARIB STD-T71 version 1.0, Association of Radio Industries and Businesses, settled in 2000).
More specifically, the method of carrier sense used includes the following two types: one is a physical carrier sense method in which the received power of a wireless channel is measured using an RSSI (Received Signal Strength Indication) or the like to detect whether or not another station is using the wireless channel to transmit a wireless packet. The other is a virtual carrier sense method in which the occupied time of a wireless channel to be used in transmission and reception of a wireless packet described in the header of the wireless packet is used to set the wireless channel to a busy status only during the occupied time.
This virtual carrier sense method will be now described with reference to an example of a wireless packet communication method which uses two wireless channels as shown in FIG. 49. The STAs have a timer for indicating a so-called NAV (Network Allocation Vector) or the time which a wireless channel takes until it becomes idle. The NAV being “0” indicates that the wireless channel is idle, while the NAV being not “0” indicates that the wireless channel is busy due to a virtual carrier sense. When one STA has received a wireless packet transmitted from the other STA, the one STA reads the occupied time described in the header of the wireless packet. If the value thereof is greater than the current value of the NAV, then the one STA sets the NAV to the value.
At this time, the actual transmission time of the wireless packet may be defined as the occupied time described in the header of the wireless packet. In this case, both the physical carrier sense by the RSSI and the virtual carrier sense by the NAV indicate a busy status, and thus the carrier sense according to the aforementioned two methods serves substantially in the same manner. On the other hand, an occupied time greater than the actual transmission time of a wireless packet may be described in the header. In this case, even after the wireless packet has been completely received, the wireless channel is made busy due to a virtual carrier sense, thereby effectively inhibiting the use of the wireless channel for transmission. As used herein, the occupied time in this case is referred to as the “transmission inhibition time”. The STA transmitting a wireless packet determines the wireless channel to be idle only when it is found idle by both of the two carrier senses, and performs transmission.
In FIG. 49, at timing t1, a wireless channel #2 has a setting of NAV, and a wireless channel #1 is determined to be idle. Accordingly, a STA 1 transmits a wireless packet to a STA 2 using the wireless channel #1. The STA 2 and other STAs receive the wireless packet transmitted from the STA 1, thereby allowing the wireless channel #1 to have a setting of NAV. This causes the wireless channel #1 to be inhibited from transmission in the STAs other than the STA 2, thereby allowing the STA 2 to transmit an ACK packet to the STA 1 using the wireless channel #1. On the other hand, at timing t2, the STA 1 and the STA 2 receive a wireless packet transmitted from another STA using the wireless channel #2, so that a corresponding NAV is defined (updated). Accordingly, the wireless channel #2 is inhibited from transmission, so that the STA 1 and the STA 2 cannot transmit using the wireless channel #2.
In the wireless packet communication utilizing multiple wireless channels assigned consecutively along a frequency axis, it is anticipated that the characteristics of a transmission/reception filter and the non-linearity of an amplifier may cause a signal transmitted in a wireless channel to leak into an adjacent wireless channel. When a received signal stays in the adjacent wireless channel suffering from the leakage, the received signal may not be successfully accepted depending on the difference between the incoming leakage power and the power of the received signal. Typically, power leakage from an adjacent wireless channel upon transmission is much greater than the received power of the wireless packet which has been transmitted from a remote STA, thus making it impossible to receive the wireless packet. When the wireless packet cannot be received, there will occur a problem as shown in FIG. 50.
It is assumed that during transmission of a wireless packet using the wireless channel #1 which is idle at timing t1, a wireless packet transmitted from another STA using the wireless channel #2 at timing t2 is scheduled to set the NAV to a longer transmission inhibition time than the transmission time thereof. At this time, an occurrence of leakage from the wireless channel #1 to the wireless channel #2 in the STA 1 would make it impossible to receive the wireless packet in the wireless channel #2 and set (update) the NAV. For this reason, in the wireless channel #2, the primary virtual carrier sense is not properly performed, so that the wireless channel #2 will be determined to be idle at the next timing t3. That is, the STA 1 cannot inhibit transmission over the wireless channel #2. On the other hand, in the STA 2, the wireless channel #2 has a setting of NAV to inhibit transmission. At this time, in the wireless channel #2, it is anticipated that the wireless packet transmitted from the STA 1 at timing t3 may collide against a wireless packet transmitted from another STA, thus resulting in reduction in throughput. Furthermore, it is difficult to live alongside the conventional wireless packet transmission method that utilizes only the wireless channel #2.
It is also anticipated that leakage into wireless channels may occur not only into adjacent channels but also into many other wireless channels such as the next adjacent wireless channels, thereby causing the virtual carrier sense not to be properly performed over a wider range.
It is an object of the present invention to provide a method and device for wireless packet communication which can reduce factors responsible for decreased throughput resulting from leakage into adjacent channels or the like in a wireless packet communication system that uses multiple wireless channels.